by Lui Chak Hing 14073317D Final Report Bachelor of...

THE HONG KONG POLYTECHNIC UNIVERSITY

DEPARTMENT OF ELECTRICAL ENGINEERING

1

Project ID: FYP_71

Simulation of virtual-current chopping in circuit breakers in

electrical supply systems

by

Lui Chak Hing

14073317D

Final Report

Bachelor of Engineering (Honours)

in

Electrical Engineering*

Of

The Hong Kong Polytechnic University

Supervisor: Dr C.W. Yu Date: 31/3/2018



2

Contents

Abstract................................................................................... 3

Acknowledgement................................................................... 4

Chapter 1:

Introduction........................................................................ 5 - 9

Chapter 2:

Objectives.............................................................................. 10

Chapter 3:

Background.................................................................... 11 - 13

Chapter 4:

Methodology.................................................................. 14 - 24

Chapter 5:

Results............................................................................ 25 - 39

Chapter 6:

Discussion...................................................................... 40 - 42

Chapter 7:

Conclusion............................................................................. 43

References...................................................................... 44 - 46



3

Abstract

Vacuum circuit breaker (VCB) provides highly enhanced current interruption

and dielectric recovery features. It has the ability to interrupt the high frequency

(HF) current effectively. In some cases, HF frequency current may superimpose

on the power-frequency current. This can cause multiple reignitions and serious

voltage escalations under some conditions. In this project, computer simulation

will be performed in order to study and analyse the properties of such process.

A mathematical VCB model which incorporates different phenomena during the

operation is constructed. These include the HF current quenching capability and

the recovery of dielectric strength of VCB. This model replicates the original

circuit breaker properties and simulate the phenomenon of the transients. The

construction of the VCB model is conducted in the simulation software EMTP-

RV. The developed model is implemented in a simple-phase and a three-phase

testing circuits. Simulation is then performed to obtain the transient behaviour

of VCB and analyse the effects on multiple reignitions phenomena under

different parameter values. The simulation work reveals that there may be a

number of multiple reignitions when the dielectric strength is low or arcing time

is short. Afterwards, the developed VCB model is implemented in a typical

electrical supply circuit. The circuit involves the switching of an arc furnace

transformer. The behaviour of virtual current chopping and the effects on

overvoltage phenomena of VCB will be analysed. Finally, suitable protective

measures such as surge protection are necessary in order to eliminate the

problem of virtual current chopping and hence the connected system can be

protected.



4

Acknowledgement

In the project, I would like to take this chance to express my thanks to my

supervisor Dr C.W. Yu.

I am very grateful to Dr Yu who offers me this challenging project. The

valuable advice given by Dr Yu has been a great help in doing the Project.

In addition, I wish to thank all of the people for their undivided support and

encouragement. They have given me lots of help and contributed greatly to the

Project.



5

Chapter 1: Introduction

Circuit breakers are widely used and have become the important equipment

for the protection of transmission and distributions systems for more than a

hundred years. They play a vital role in clearing faults and isolating

defective parts of network clearly and rapidly. They also take the role of

normal load and fault switching [1]. In the early days, the design of circuit

breaker is very simple and air is used as an arc interruption medium.

Starting from the 20th century, there was a rapid development of oil-filled

and air-blast forced cooling circuit breaker. Later, the development of SF6

and vacuum circuit breaker (VCB) started and has been in use. When

compared to early designs, the improved design of circuit breaker can

provide the advantage of reduction of chamber volume and improvement of

the dielectric features. Nowadays, oil-insulated and gas-insulated circuit

breakers are still irreplaceable for high voltage levels. For medium voltage

levels, VCB have dominated the switching functions in power systems.

A circuit breaker can be viewed as one moving and one fixed contact. These

contacts are either placed in a special container which contains the particular

extinguishing medium (e.g. gas and oil) or a vacuum bottle. The contacts are in

closed state under normal operation and current will flow through the device

without major losses. If a signal of opening is sent to the circuit breaker, the

contacts are separated by an external mechanism. When the separation of

contacts starts, current still continues to flow between the contacts as an electric

arc. The presence of electric arc is due to the supplied energy and the arc will be

present until the energy is eliminated by some ways. In an AC system, there are

two zero-crossing points for current in every cycle. Circuit breaker can have the

chance to extinguish the electric arc in these current zeros because of the zero

energy input in these points. The design of a circuit breaker should fulfil certain

criteria such as interrupting at natural current zeros, meeting the thermal

interruption requirements and withstanding dielectric stresses caused during the

interruption process.



6

Figure 1.1 - A typical VCB

Out of many types of circuit breakers, VCB (shown in Figure 1.1) are

commonly adopted in power systems with medium voltage range due to many

advantages, such as small size, less maintenance, excellent performance on the

interruption and recovery of dielectric strength [2]. VCB can interrupt current

with a very high value of 𝑑𝑖

𝑑𝑡 , ranging from 150A/µs to 1000A/µs typically [3].

However, everything has two sides. The switching transient overvoltage

phenomenon due to repeated reignitions, current chopping and virtual current

chopping are commonly found in VCB [2].

Figure 1.2 - Current chopping



7

When VCB is opening, electric arc arises and the arc will extinguish at a natural

current zero ideally. When the arc conducts a small current, the arc becomes

unstable and may disappear before natural current zero [2]. The above process

is described as current chopping as shown in Figure 1.2 and is commonly found

in VCB during the interruption of inductive and capacitive current [1].

When current chopping happens, the transient recovery voltage (TRV) arises

between the gap of VCB. Once the level of TRV is higher than the level of

dielectric strength of vacuum gap, reignition will happen and consequently arc

will appear again [3]. The process will cause a flow of HF current due to the

stored charges in the stray capacitance on either side of the gap of the breaker.

The power-frequency current can be superimposed by the HF current, as shown

in Figure 1.3 [4].

Figure 1.3

VCB can interrupt the HF current at one of the current zeros. When the

interruption occurs, the process of multiple reignitions may happen and it may

produce undesirable voltage surges [3]. Voltage escalation will continue until

the contact separation of VCB is enough to withstand the TRV or the magnitude

of power-frequency current is high enough to prevent zero crossing of the sum

of HF current and power frequency current. In this situation, there is no

interruption and the current will continue for one more half cycle [1]. This



8

process can repeat several times. If the HF current of one phase is flowing into

other two phases of the system through the electrical couplings of the load, the

HF current can superimpose on the power-frequency current of these phases and

force the power-frequency current to zero. Virtual current chopping happens

and this will lead to severe overvoltage in three phases [2].

It is reported that a number of transformer insulation failures have occurred in

the systems with the installation of VCBs. This is possibly due to the switching

actions of VCBs, although these transformers have complied with associated

requirements and passed all standard tests previously [5]. A study on the

investigation of transformer failures revealed that the HF transients are the

major cause of the insulation failures (around 34%) [6].

The switching transients modelling in VCB is important in order to analyse the

overvoltage behaviour which may happen at different stages. However, the

accurate properties and performance of VCB are difficult to simulate because of

the confidential information of suppliers and the limitation of experiments [2].

A number of VCB models exist and the factor of arc thermal instabilities is

considered. However, there is currently no universal precise arc model due to

the complexity of the arc physics [3].

The simplest circuit breaker model is assumed to have zero impedance in closed

state and infinite impedance in open state. The breaker will open at the first

current zero after giving the corresponding signal and TRV of the breaker can

be obtained by using this model. The consideration of arc (time-varying) is

needed in more complicated models. This requires the parameters of arc which

are sometimes difficult to obtain. For advanced models, the circuit breaker can

be represented as a dynamically varying conductance or resistance which is

determined by the past values of voltage and current in the arc.



9

In this project, modelling of VCB will be performed. Then simulation of virtual

current chopping will be done and the effects of overvoltage transients in

electrical supply system will be analysed.

This project is organised by the following sequence.

Chapter 1 is the introduction of the project, including the history of circuit

breaker and the transient behaviour of VCB.

Chapter 2 is the objective of the project, which will introduce what outcomes

can be introduced after finishing the project.

Chapter 3 will mainly provide different research ideas proposed by different

researcher in the area of circuit breaker.

Chapter 4 introduces the methodology that is mainly about how the VCB model

is constructed and what simulation tasks are required in the project.

Chapter 5 mainly presents the results of the simulation in different circuit

configurations.

Chapter 6 will mainly provide explanations of the simulation results and some

extensive ideas

Chapter 7 concludes the whole project.



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Chapter 2: Objectives

In this project, several objectives would like to be achieved, including

Developing a VCB model incorporating different parameters of the circuit

breaker in the simulation software EMTP-RV

Performing the simulation in the single-phase and three-phase testing

circuits

Analysing the simulation results and investigating the transient behaviour

of VCB with different parameters

Applying the VCB model to a practical circuit which is related to the

switching of an arc furnace transformer

Simulating the virtual current chopping process and analysing some

protective measures which can reduce the overvoltage and reignitions

behaviour of VCB



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Chapter 3: Background

The problems that arise from the switching of VCB have been a major research

subject for many years. A lot of work has been conducted on the analysis of

physical behaviour regarding the arc circuit interaction and current interruption.

The effect that VCB has when switching different networks boosted the

research after the commercial application of VCB in the past decades. In the

early days, the research was focussed mainly on the high level of chopping

current of VCB. Greenwood [7] performed the research work on explaining

how chopping current reacts with motor and transformer circuits. Later, the

behaviour of current chopping was further analysed. For example, Czarnecki

and Lindmayer [8, 9, 10] conducted a research on the effects of contacting

materials on VCB interrupting performance around current zero. From the result

of the research, an expression for the chopping current value was obtained. This

was a difficult task due to the statistical nature of current chopping phenomena.

Damstm and van den Heuvel [11] and Gibbis et al. [12, 13] later performed the

work on comparing the chopping current levels of various SF6 breakers with

those of VCBs. The results showed that the levels of VCB chopping current is

distributed randomly with a value from a range that is dependent on the types of

contacting materials. Smeets [14] conducted a further research on the behaviour

of low current arc. A more generally applicable expression for evaluating the

level of chopping current was obtained.

Researchers have done quite a lot of studies on VCB modelling and overvoltage

transient behaviour. A wide range of work are studied and analysed in this

research area. In recent years, some researchers have developed a mathematical

VCB model that comprises the random characteristics of various phenomena

that take place in the VCB operation, including the arcing time, recovery of

dielectric strength, quenching capability of HF current and current chopping

level of the VCB [2]. Different combinations of these properties were chosen

and the performance of a number of breaker models were analysed. From this, a

brief idea can be obtained on how to model VCB properly by considering these

characteristics.



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A model of single-phase VCB was presented in an academic paper [15],

including the factors which have the effects on overvoltage produced as

reignitions happen during the opening of VCB. These included some circuit

parameters and breaker characteristics (e.g. interrupting capability and rate of

change of dielectric strength, etc.). Some random events e.g. arcing time were

also included. It had the assumption that the dielectric strength of VCB

increases with time linearly. The VCB model was constructed to analyse these

factors. The model was verified by measuring current and voltage values across

different VCBs in a testing circuit.

A model of three-phase VCB was developed in the simulation software in

another paper [1]. The model incorporated some characteristics such as arcing

time, chopping current level, quenching capability of current and dielectric

strength between the contacts. Some were evaluated based on statistical

approach such as using a normal distribution with a certain standard deviation.

A research [16] was conducted on the simulation of voltage escalation and the

behaviour of reignition in VCBs when they are generator circuit breakers. Nine

particular models of VCB were investigated. The model include an RC branch

parallel to the breaker to represent the gap stray capacitance. Similar to other

research papers, the VCB features were modelled by linear expressions for

dielectric strength of the breaker and HF current quenching capability. The

research showed that multiple restrikes in the process may generate a voltage

escalation when the arcing time is short (e.g. 0μs – 100μs).

Other scholars have done similar research work and studied the transient

behaviour of VCB in different practical situations. Some examples include the

shunt reactor and arc furnace transformer switching [1] [4]. These studies can

provide some directions and ideas on this project.



13

Some researchers [2] conducted the study on transient behaviour which happen

at different parts within an offshore wind farm. The models for transformers,

cables, wind turbines and circuit breakers are reviewed.

The problem of motor switching have also attracted great interest because

vacuum contactors are widely applied in motor circuits. The overvoltage

phenomena, which result from multiple reignitions, current chopping and virtual

current chopping, can vary greatly depending on different motor conditions (e.g.

unloaded, normal load or starting up). By considering different motor circuits,

analysis of motor switching can be performed. These studies were performed by

a number of researchers such as Smeets et al. [17] and Smeets [18, 19].

Besides the energisation and de-energisation of motors, capacitive loads can

cause overvoltage phenomena as well. Pu and Damstm [20] conducted a study

that combined the computer simulation and experiments on capacitive load. The

results showed that the actual phenomena are affected by both the breaker

parameters and the network

Overvoltage transient of VCB can lead to transformer failures [5]. Some studies

about this issue have been analysed and presented [5] [21]. This research area is

important in studying transformer protection. Various simulation work and

testing have been performed by researchers [22]. The background information is

useful when doing the related work in this project.



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Chapter 4: Methodology

The project will be mainly conducted in the simulation software EMTP-RV.

With the help of the software, complex systems can be simulated and analysed.

The detailed properties and phenomena of VCB are discussed in this section.

Afterwards, a VCB model that includes the characteristics of current chopping,

voltage escalations and multiple reignitions will be introduced [3]. A

mathematical VCB is then constructed in the software environment. The

parameters of the breaker are obtained from different research papers.

When switching a VCB, transient phenomena may happen in different

situations. Even for the small differences in the circuit parameters values can

lead to large differences in the final results. The overvoltage behaviour in VCB

is random in nature and it is dependent on the actual configuration of the

system, breaker conditions and associated parameters. Various ways can be

employed for analysing the overvoltage behaviour and voltage oscillation

generated during the switching of VCB. These include theoretical quantitative

analysis, laboratory/field testing and computer simulation.

For laboratory testing, one major problem is the difficulty of getting a realistic

load model. Also, it has the difficulty in constructing the testing circuits that

match the practical situations. For field test, the drawbacks are difficult to

perform and expensive. There is a possible risk that the circuit breakers and

circuit components may damage. For theoretical analysis, it can be a useful tool

to obtain insight in the problems related to VCB switching. However, it cannot

model the circuit breaker and system in detail. Some models (e.g. transformers

and cables, etc.) are frequency dependent. A lot of time may be consumed to

analyse the complicated network. Nowadays, computer simulations are widely

used for the modelling of different complicated systems.

An appropriate VCB model is essential in order to simulate the transient

behaviour in electrical system. However, the accurate breaker model is very

difficult to obtain because of the limited information and the complicated



15

breaker behaviour during operation [2]. Therefore, some researchers have

proposed the stochastic model [3].

When the VCB is opening, the dielectric strength of the contacts increases with

time as well. When the TRV is higher than the dielectric strength of VCB

contacts, reignition will happen and the model will generate a closed signal.

When the rate of change of HF current at a zero crossing is lower than the

quenching capability of HF current of VCB, the model will send an opening

signal to the switch [2]. The HF current will be interrupted. If multiple

reignitions happen, the above-mentioned process will repeat until the dielectric

strength of VCB is able to withstand the TRV.

The general VCB model includes some characteristics inherent to the VCB

operation to control the actual behaviour of the breaker during the process of

simulation [1]. These characteristics include the current chopping level, arcing

time, recovery of dielectric strength and HF current quenching capability [3].



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Arcing time

Arcing time is the time interval between the opening of VCB contact and the

subsequent current zero.

Chopping current level

Current chopping is the phenomenon that the power-frequency current is

suppressed before its natural zero crossing in the breaker. If the capacitive or

inductive current is interrupted, the arc appears and power-frequency current

conducts through the arc [3]. When the power frequency current of first pole to

clear reaches to the low level, the arc becomes highly unstable or even

disappear. The power frequency current will be chopped before its natural zero

crossing. Current chopping is a major disadvantage of VCB because it will be

accompanied with the transient overvoltage which will affect the load side due

to oscillations [2]. The chopping current is non-deterministic. Some researchers

have proposed different mean chopping levels for various situations [3]. In the

simulation, the chopping current is calculated by [3]:

𝑖𝑐ℎ = (𝜔 ∙ 𝑖 ∙ 𝛼 ∙ 𝛽)𝑞

where:

ω = 2 ∙ π ∙ 50Hz

i = amplitude of the 50Hz current

α = 6.2 ∙ 𝑒−16𝑠

β = 14.3

q = (1 − 𝑞)−1

The chopping currents calculated from the formula are similar to those of

present VCB which use Cu/Cr as the contact materials [3]. The current

chopping level is dependent on the moment of separation of the VCB contacts

(the closer the contact opens to current zero, the higher the chopping level).



17

In the simulation, current chopping is not included in the VCB model because it

is assumed that multiple pre-strikes and re-strikes will lead to serious

overvoltage. Also, the current chopping level becomes relatively low after the

development of modern VCB.

Recovery of dielectric strength

In general, two breakdown mechanisms exist in VCB, namely the cold gap

breakdown and the hot gap breakdown. In this project, the cold gap breakdown

is considered. Researchers have shown that there is a linear relationship

between the dielectric strength value and the contact distance. The equation is

given as follow [3]:

U = A(t − 𝑡𝑜𝑝𝑒𝑛) + 𝐵

where:

t: software internal time

𝑡𝑜𝑝𝑒𝑛: VCB opening time



18

Quenching capability of HF current

The power-frequency current is superimposed by the HF current if reignition

happens. The HF current has some zero crossings and the breaker has the

capability to extinguish the HF current in one of the zero crossings [3]. The rate

of change of HF current at zero crossing determines whether the current can be

interrupted by the VCB or not.

In earlier study, the quenching capability of HF current is modelled by [3]:

𝑑𝑖

𝑑𝑡= 𝐶(𝑡 − 𝑡𝑜𝑝𝑒𝑛) + 𝐷

where:

𝑑𝑖

𝑑𝑡: rate of change of the current

C: slope of the equation

t: software internal time

𝑡𝑜𝑝𝑒𝑛: VCB opening time

D: intercrpt of the equation

When the rate of change of HF current in one zero crossing is lower than the

calculated di/dt value, the HF current will be extinguished.



19

Figure 4.1a and 4.1b show the flow charts of opening and closing operation of

the VCB model.

Figure 4.1a - Flow chart for VCB opening operation



20

Figure 4.1b - Flow chart for VCB closing operation



21

Figure 4.2 - A controlled switch

The VCB model is characterised by a controlled switch (shown in Figure 4.2)

and the switch will be open or closed based on a specific mechanism. The state

of the VCB is determined by voltage, current and the previous state of the

breaker. The flow charts for VCB opening and closing operation are shown in

Figure X

For the closing operation, the dielectric strength of the VCB will start to

decrease. Once the VCB voltage exceeds the dielectric strength of the breaker at

a particular time, a closing is generated. If the slope of the current at zero-

crossing point is lower than the HF current quenching capability, an opening

signal is generated and hence the pre-strike is interrupted. The process will

continue until the dielectric strength of the breaker reduces to zero.

When the VCB starts to open, the dielectric strength of the breaker will begin to

increase with time. If the slope of the current at the zero-crossing point is lower

than the HF current quenching capability of the breaker, the switch will be



22

open. TRV will appear across the contacts of the VCB. If the TRV exceeds the

dielectric strength of the breaker, the switch will be closed and hence a

reignition is simulated. The process will continue until one of the following

conditions happen:

The VCB can interrupt the current successfully when the TRV is smaller

than the dielectric strength of the breaker.

The VCB cannot interrupt the HF current after the last reignition. The

current interruption is accomplished at the next power-frequency current

zero when the dielectric strength is higher.

The VCB cannot achieve the interruption in any period.



23

Figure 4.3 - EMTP-RV software

Figure 4.4 - VCB model

Figure 4.4 shows the developed VCB model that is implemented in the EMTP-

RV environment.

In this project, computer simulation is performed on different network

configurations by using the simulation software EMTP-RV. These

configurations include the testing circuits of single phase and three phases. An

application circuit which is about the switching of an arc furnace transformer is

also simulated. By using the simulation results, transient behaviour of VCB can

be analysed under different situations. Figure 4.5, 4.6 and 4.7 show the different

network configurations.



24

Figure 4.5 - Single-phase testing circuit

Figure 4.6 - single-phase testing circuit

Figure 4.7 - Switching of a transformer



25

Chapter 5: Results When a load is disconnected by the circuit breaker, one of the situations may

occur in general:

The load current is interrupted successfully by the VCB and reignition

will not occur.

Multiple reignitions occur and VCB interrupts the HF current. After the

process is repeated for several times, the VCB withstands the transient

recovery voltage and interrupt the current successfully.

The VCB cannot interrupt the HF current and power-frequency current

conducts through the arc. Successful interruption is extended in the next

zero point of power frequency current.

The VCB cannot interrupt the current in any position of the period.

Serious damage to equipment may occur.



26

Simulation of single-phase cases

Testing circuit

Figure 5.1 - Single-phase testing circuit

Circuit parameters



27

Results

Case 1

Data:

Recovery of dielectric strength A = (50V/µs)

Quenching capability (D = 100A/µs)

Waveforms:

Figure S1 C1-1 - Voltage across the contacts of VCB

Figure S1 C1-2 - VCB current



28

Figure S1 C1-3 - Voltage on the load side

The results are shown in the above figures. The increase of TRV is always slower

than the increase of dielectric strength of VCB. As a result, the VCB can

withstand the TRV and interrupts the load current successfully. When the current

chopping occurs in the VCB, a transient overvoltage with several kHz oscillations

appears in the load because the energy stored in LL is transferred into CL.



29

Case 2

Data:

Recovery of dielectric strength (A = 20V/µs)


Waveforms:


The TRV exceeds the dielectric strength of VCB at a particular time. The first

reignition occurs. The voltage across the VCB starts to have a high frequency

oscillation after the zero power-frequency current. The high frequency

oscillation is induced from the interaction of CL, CS and LK. The expression is

given by:

𝑓1 =1

2π√𝐿𝑘𝐶𝑠𝐶𝐿

𝐶𝑠 + 𝐶𝐿

= 1.8𝑀𝐻𝑧

The above HF voltage oscillation will be damped after very short time. The

frequency of next voltage oscillation is much lower and can be determined by:

𝑓2 =1

2π√𝐿𝐿𝐶𝐿

= 4.6𝑘𝐻𝑧

It represents the natural frequency of the load.



30


Figure S1 C2-3 - VCB current of the first HF oscillation

Figure S1 C2-4 - VCB current of the second HF oscillation

As a direct result of reignition, the HF current is injected into network and two

HF oscillations can be observed. The first HF oscillation is due to the

interaction of CS and LS. The frequency can be determined by:

𝑓3 =1

2π√𝐿𝑠𝐶𝑠

= 50𝑀𝐻𝑧

This HF current is damped quickly and it is not interrupted in its zero point by

the VCB. The second HF current is caused by the interaction of LK and CL. The

frequency can be determined by:

𝑓4 =1

2π√𝐿𝑘𝐶𝐿

= 0.25𝑀𝐻𝑧

Figure S1 C2-5 - Voltage on the load side



31

Case 3

Data:

Recovery of dielectric strength (A = 20V/µs)


Waveforms:



Figure S1 C3-3 - VCB current of the first HF oscillation

Figure S1 C3-4 - VCB current of the second HF oscillation



32

The quenching capability of HF current of VCB plays a vital role in interrupting

the second HF current. When the slope of the HF current at current zero is smaller

than the HF current quenching capability of the VCB, this HF current will be

interrupted. In this case, the HF quenching capability is higher (D = 200), the

VCB can interrupt the HF current earlier when compared to D = 100. As a result,

TRV will occur earlier so the chance of another reignition will increase because

the dielectric strength of the VCB is lower in the early time. The arc will then be

extinguished earlier and hence the chance of having HF current zeros will

increase. The process will continue until the power frequency current increases

to a level where there are no current zeros with the slope of 200 or less. The arc

will continue to exit until the time of second current zero crossing, when the

dielectric strength of the VCB can withstand the TRV.



33

Simulation of three-phase cases

Testing circuit

Figure 5.2 - Three-phase testing circuit

Circuit parameters



34

Results

Case1

Waveforms:

Figure S2 C1-1 - Voltage across the VCB contacts at phase A

Figure S2 C1-2 - Voltage across the VCB contacts at phase B

Figure S2 C1-3 - Voltage across the VCB contacts at phase C

Figure S2 C1-4 - VCB voltages of all phases



35

Figure S2 C1-5 - VCB current at phase A

Figure S2 C1-6 - VCB current at phase B

Figure S2 C1-7 - VCB current at phase C

Figure S2 C1-8 - VCB currents of all phases

Figure S2 C1-9 - Load voltages of all phases

The results of the simulation show that the reignitions phenomena is not serious

and the VCB can withstand the TRV after few cycles.



36

Case 2

Waveforms:


Figure S2 C2-2 - VCB current of all phases


The results of the simulation show that there is a serious and multiple

reignitions in this VCB when compared to the previous case. High frequency

oscillations appear in the current waveforms of the VCB. The circuit breaker

can only withstand reignitions after a number of cycles.



37

Simulation of the circuit involving switching of a transformer

Testing circuit

Figure 5.3 Switching of an arc furnace transformer



38

Results

Case 1

Description:

The circuit has no surge protection.

Waveforms:






39

Case 2

Description:

The circuit has a surge arrestor with the rating of 35.35kV.

Waveforms:






40

Chapter 6: Discussion

The simulation works are mainly based on three circuits, including a single-phase

and a three-phase testing circuit. In addition, an application circuit involving the

switching action of an arc furnace transformer is simulated as well.

For the simulation of single-phase testing circuit, different cases are involved in

order to investigate the VCB behaviour during the opening process. Different

parameters (dielectric strength of VCB and HF current quenching capability) are

set to analyse to effects on reignitions phenomena and HF current property of

VCB. The results show that overvoltage and multiple reignitions are more likely

to occur when the dielectric strength is smaller or the HF current quenching

capability is higher. Different frequency values are evaluated and this shows that

there are high frequency oscillations during the process of reignitions in VCB.

For the three-phase testing circuit, similar simulation tasks are performed. The

results show that VCB experiences reignitions at different phases under various

time intervals. The HF current produced at one phase will through other phases

and force the power-frequency current to zero. The forced current zero in that

phase will cause TRV in the breaker. As a result, further reignitions will produce

and this will lead failure of the VCB.

Sensitivity analysis of parameters is important to find the trend which would

cause serious multiple reignitions and escalation of voltage in the circuit breaker.

According to a research paper [3], 48 combinations of VCB parameters are

analysed.

The paper gives a summary on the effect of the quenching capability of HF

current and the recovery of dielectric strength with different ranges of arcing time

on voltage escalation. When the recovery of dielectric strength is in middle range

(between 20V/µs and 30V/µs), voltage escalation is usually higher than other

cases. For arcing time, when it is small (0µs - 100µs), voltage escalation is more



41

serious than those arcing time ranging from 100µs to 300µs. High voltage

escalation also usually occurs when the slope of HF current quenching capability

is negative. From the result, some combinations of the parameters may cause

reignitions and voltage escalation. This may happen when the recovery of

dielectric strength is in middle range, the arcing time is short or the quenching

capability of HF current is decreasing with time.

The results show that the arcing time and the recovery of dielectric strength are

the most important parameters for the estimation of TRV. The effect of the

quenching capability of HF current on voltage escalation is less noticeable. The

illustration figures are shown in Figure 6.1.

Figure 6.1



42

The VCB model developed in this project is put into a practical circuit for the

purpose of simulation. It involves the switching of an arc furnace transformer. A

lot of switching operations are needed during the operation of the transformer.

VCB is a suitable type of circuit breaker for this application due to the cheap

maintenance cost and the capability of switching high current at lots of switching

cycles. However, switching of transformer may lead to high stress on the network.

In the circuit, the phenomena of virtual current chopping can cause the failure of

the VCB and other devices. Therefore, adding the RC surge suppressor or surge

arrestor is necessary. The process of virtual current chopping can be simulated by

using the developed VCB model. Suitable values of the surge suppressor and

surge arrestor can be found to minimise the overvoltage effect.

In the simulation of the practical circuit, Case 1 is the normal loading without

adding surge arrestor. The results of the simulation show that reignitions

happened during the operation of the circuit breaker. Hence, this caused serious

virtual current chopping. Case 2 is the circuit with the addition of surge arrestor

which has the rating of 35.35kV. After adding the surge arrestor to the circuit, the

overvoltage behaviour was limited and hence the phenomena of virtual current

chopping were improved greatly. As a result, the addition of surge arrestor can

help to protect the system.



43

Chapter 7: Conclusion

In conclusion, the characteristics of VCB and phenomena of transient overvoltage

are discussed. A VCB model is built and implemented in the simulation software.

A single-phase and three-phase testing circuit is developed to perform the

simulation with different parameters and observe the phenomena of multiple

reignitions and voltage escalations of VCB.

Moreover, the results of sensitivity analysis show that the parameters of arcing

time and the recovery of dielectric strength play a significant role for the

occurrence of multiple reignitions and overvoltage. The quenching capability of

HF current has less effect. Consequently, when estimating the overvoltage

phenomena of VCB, it is suitable to consider the recovery of dielectric strength

in the middle range with smaller arcing time to look for the worst situation so that

the appropriate protection scheme can be provided.

The developed VCB model is implemented in an application network which

involves switching of a transformer. The phenomena of virtual current chopping

is observed in the process of simulation. One possible way of reducing the

problem of virtual current chopping is the addition of the RC surge suppressor or

surge arrestor. This can reduce the chance of equipment failure and hence protect

the system.



44

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45

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46

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